Electromagnetic forming is a method of forming sheet metal or thin walled tubes that is based on placing a work-coil in close proximity to the metal to be formed and running a brief, high intensity current pulse through the coil. If the metal to be formed is sufficiently conductive the change in magnetic field produced by the coil will develop eddy currents in the work piece. These currents also have associated with them a magnetic field that is repulsive to that of the coil. This natural electromagnetic repulsion is capable of producing very large pressures that can accelerate the work piece at high velocities (typically 1-200 meters/second). This acceleration is produced without making physical contact to the work piece. The electrical current pulse is usually generated by the discharge of a capacitor bank. This field has been developed by many individuals and companies and is widely used for the forming and assembly of tubular and sheet work pieces. Several excellent reviews of the field are available, including Moon, F. C., Magneto-Solid Mechanics, ASTME, High Velocity Forming of Metals, revised edition (1968); Plum, M. M., Electromagnetic Forming, Metals Handbook, Maxwell Laboratories, Inc., pp. 644-653; and Belyy, I. V., Fertik, S. M. and Khimenko, L. T., Electromagnetic Metal Forming Handbook, Khar'kov State University, Khar'kov, USSR (1977) (Translation from Russian by M. M. Altoynova 1996), all of which are hereby incorporated herein by reference. Examples of prior art patents involving electromagnetic forming include U.S. Pat. No. 4,947,667 to Gunkel et al., U.S. Pat. No. 4,531,393 to Weir et al., U.S. Pat. No. 5,353,617 to Cherian et al., U.S. Pat. No. 3,998,081 to Hansen et al., U.S. Pat. No. 5,331,832 to Cherian et al., U.S. Pat. No. 5,457,977 to Wilson, U.S. Pat. No. 4,619,127 to Sano et al., U.S. Pat. No. 4,473,862 to Hill, U.S. Pat. No. 4,151,640 to McDermott et al. and U.S. Pat. No. 5,016,457 to Richardson et al., all of which are hereby incorporated herein by reference.
Electromagnetic forming can be carried out on a wide range of materials and geometries within some fundamental constraints. First, the material must be sufficiently electrically conductive to exclude the electromagnetic field of the work-coil. The physics of this interaction have been well characterized.
It is an object of the present invention to provide apparatus and methods that take advantage of such actuators and to use them in conjunction with, mold and tool bodies.
Although not limited in their application to the automobile industry, many of the problems solved and advantages achieved with the apparatus and methods of the present invention can be appreciated by reference to the problems faced in the forming of sheet metals in that industry.
The automotive industry is currently interested in producing automobile body parts from aluminum alloys. The weight saving of up to 50% of the body-in white and its attendant gains in fuel efficiency are largely responsible for this interest. Additionally, the superior recycle characteristic of aluminum is recognized as becoming of increasing importance as the total life cycle cost of automobiles becomes an issue. [Du Bois 1996, Henry 1995]
The press forming of aluminum alloys have problems in comparison to steel principally due to very low strain rate hardening, low r (strain ratio) value and high galling tendency. In particular the lack of strain rate hardening behavior in aluminum alloys at room temperature is troublesome since this is the characteristic that allows post uniform plastic strain in a sheet metal. All good draw quality sheet steels have enhanced strain rate sensitivity which is identifiable by a long arching stress-strain curve. The press forming handicap of aluminum alloys, measured by the lack of strain rate sensitivity, is shown by the direct comparison of the stress-strain curves for typical auto body steel and aluminum sheet FIG. 10 which was adapted from an Aluminum Association report [Al Assoc.,1996].
Despite the press working "fussiness" of aluminum, car builders are currently using aluminum for selected body panels such as hoods outer door skins and trunk lids. These are parts that are geometrically simple and can be stretch-draw formed with conventional matched tools. However, the propensity of aluminum alloys to neck and tear at relatively low strain levels, makes many of the more geometrically complex body parts extremely difficult or impossible to produce in aluminum with conventional matched tools. A side-by-side comparison of two automobile door-inner panels from the same stamping die was conducted to manifest the material characteristics shown in FIG. 10. A filly formed panel of specified production steel sheet that was produced after set-up trials indicated satisfactory tool performance. A second panel of 6111-T4 aluminum of the same gauge as the steel was processed directly after the steel panel. The aluminum panel showed wrinkling and large splits that occurred within the first 25% of the tool stroke, which was not unexpected.
Fluid pressure forming methods such as Verson-Wheelon, ABB or Hydroform can extend the formable geometry for aluminum sheet somewhat but at the cost of long cycle time leading to unacceptably low production rates. Fluid pressure methods have high capital equipment costs compared to conventional press machines due principally to the high static operating pressures.
Several aluminum alloy exhibit superplastic creep behavior which can be utilized to produce very complex sheet part geometries. Current superplastic forming methods also suffer from inherently long cycle times in addition to requiring high temperatures and specialized alloys. Control of superplastic forming is inherently more complex in that it requires the explicit control of worksheet temperature and forming gas pressure during the forming cycle. The capital costs equipment costs are also significantly greater than the conventional [Laycock,1982].
A compromise solution might be to change the part designs to shapes which can be produced in aluminum using current production methods. Another solution would be a new sheet forming method which could overcome the formability short-comings of aluminum alloys while maintaining acceptable production rates (150-300 parts/hr. for large body panels). Such a processes would be less restrictive for the automobile designers and thus more appealing to the industry. In addition, this improved forming performance must be attainable with capital equipment and tooling expenditures which will maintain competitive production part costs. To this end, it would be an added advantage if this new method could actually provide a reduction in tooling costs compared to current practice. Such a cost reduction may be attainable if, for instance, the new method required only a single part-surface tool instead of a precisely matched pair. Single-sided form tools, currently used in the fluid forming processes need fewer trials and subsequent geometry alterations before producing good parts. Another highly beneficial attribute of the new process would be implementation using the installed press machines that are currently used by the industry for conventional sheet metal stamping.
Hypothetically, a method that would completely fulfill the performance criteria listed above might be designed using a "clean sheet" approach. However it is quite likely that many of the attributes of current processes would be re-invented. Most complex technologies emerge in a evolutionary manner, incrementally with occasional forward leaps. Therefore, an examination of existing methods for evidence of partial solutions to the total problem is appropriate.
It is therefore an object of the present invention to produce hybrid apparatus and methods that go further toward meeting the ideal performance goals than the prior art devices and methods.
The existing processes of interest as components of a combined hybrid method are; conventional matched tools, fluid pressure processes and the high velocity, impulse power processes. The common characteristic that these methods share is a general insensitivity to alloy type or inherent restriction of forming rate. Superplastic forming has been omitted under this same rational, although near term developments in superplastic forming may indeed increase its viability as a production method for aluminum auto body panels. Each of the included methods have a significant track record in some production niche and have attributes which are partial solutions to the overall problem of production stamping of aluminum alloy sheet. In the interest of clarity, the characteristics of these methods are briefly described below. If more detailed information on these constituent methods is desired, the reader is referred to any good text or handbook of industrial metal forming practice [e.g. Lange, 1985, Lascoe,1988].